Recently, a magneto-optical disk has drawn much attention as a high density recording medium. The magneto-optical disk typically has a structure including a protective coating layer, a magneto-optical recording layer, a protective layer, a reflection layer and a protecting coating on an optically transparent substrate having concentric circular pre-grooves on its surface. The substrate typically comprises an optically transparent material such as polycaorbonate or the like. The magneto-optical recording layer is formed by thin films comprising amorphous metal alloy (for example, TbFeCo) of rare-earth metal and transition metal having the perpendicular magnetic anisotropy photo magnetic effect. The protective coating layer comprises a silicon nitride film or the like. The protective coating layer is necessary for preventing humidity. The reflecting layer is a thin film comprising an aluminum or an alloy consisting essentially of aluminum. The reflecting layer is employed in order to improve the detection efficiency of producing signals and to control the thermal diffusion. The protective coating is formed from a UV-ray setting resin comprising a photo polymerization initiator, a photo polymerization prepolymer and a (meth-)acrylic ester. The protective coating prevents humidity from effecting the magneto-optical recording layer.
A recording apparatus writes information signals on the magneto-optical disk. The recording apparatus comprises an optical pick-up apparatus irradiating the magneto-optical disk with a laser beam, a magnetic field source apparatus applying the outside magnet field on the magneto-optical disk, a mechanism for rotating the magneto-optical disk, a means for moving the optical pick-up apparatus and a mechanism for moving the magnetic field source device.
The optical pick-up apparatus comprises a light source such as a semiconductor laser and an optical device converging light beams emitted from the light source. This optical pick-up apparatus is located at a predetermined distance from the disk and irradiates the converged light beams onto the magneto-optical recording layer of the magneto-optical disk. The light beams are transmitted through the substrate and irradiated from the back side of the main surface of a substrate of the magneto-optical disk on which the magneto-optical recording layer is formed. In the case where the output beams are demodulated by means of the optical pick-up apparatus, the output emitted from the semiconductor laser is demodulated by means of the driving circuit or the like. The method is called the "light pulse length recording method", in which information signals are written by demodulating the output of the light beam.
The magnetic field source apparatus is typically a magnetic head apparatus comprising a coil for generating a magnet field and a magnetism core to which the coil winds. This magnetic head apparatus is located adjacent to the magneto-optical disk and applies the outside magnetic field to the surface of the magneto-optical layer of the magneto-optical disk from the perpendicular direction. The outside magnetic field is applied from the main surface of the substrate of the magneto-optical disk on which the magneto-optical recording layer is formed. Where the intensity and direction of the outside magnetic field is demodulated with the magnetic head, the electric current supplied to the coil for generating a magnetic field is demodulated by means of the driving circuit or the like. This method is called the "magnet field modulation recording method", in which the information signals are written by modulating the intensity and direction of the outside magnetic field.
The magnet field modulation recording method is superior to the light pulse length recording method in terms of the following points: initialization of arranging the direction of the magnet of the recording film before writing information signals is not necessary; information can be overwritten; and the mark length recording can be done.
The method of direct overwriting information by the light pulse length recording method has been suggested. However, the structure of such magneto-optical disk is complicated because the magneto-optical recording layer comprises a plurality of thin films, thus making the manufacturing process complicated. On the other hand, where the magnet field modulation recording method is employed, direct overwriting can be easily conducted by the means of simple structured magneto-optical disk. The method of writing information by the magnet field modulation recording method is being widely used.
As stated above, the magnet field modulation recording method includes the steps of scanning the converged laser along the pre-groove of the magneto-optical disk; modulating the outside applied magnetic field and changing the direction of magnetization of the portion on which the laser of the magneto-optical recording layer is irradiated. The necessary magnet field to inverse the magnetization of magneto-optical recording layer is relatively high frequency (higher than several hundreds kHz) so as to change the direction and intensity. The generation of such a magnet field is limited to the area adjacent of the magnetic core of the magnetic head apparatus. Consequently, the distance between the laser irradiating section of the magneto-optical recording layer and the magnetic head apparatus section needs to be shortened.
As the means of setting the magnetic head apparatus in this situation, the magnetic head is rotated and placed directly on the magneto-optical recording layer. In other words, the magneto-optical disk is made to be rotating while the pressure welding the magnetic head apparatus onto the magneto-optical disk by means of springs or the like so that the magnetic head apparatus is slided into contact with the magneto-optical disk. Consequently, the distance between the magneto-optical disk and the magneto-optical recording layer is always constant and the distance becomes nearest to the magneto-optical recording layer.
In order to apply the outside magnetic field to the magneto-optical recording layer of the magneto-optical disk, the magnetic head apparatus is slided into contact with the magnetic-optical disk. However, the frictional force generated between the protective coating of the magneto-optical disk and magnetic head apparatus causes a charge increase and breaks the magnetic head and protective coating due to abrasion. Therefore, some means for decreasing the friction force between the protective coating and the magnetic head are reduced.
An example of these conventional techniques is disclosed in Japanese Laid Open Patent No. (Tokkai-Hei) 05-307777. In this method, the lubrication layer comprising silicon oil is formed on the protective coating of the magneto-optical disk. An application member, such as nonwoven cloth including the lubricant diluted by a solution is put on the surface of the magneto-optical disk on which the protective coating is formed and pressed by the pressing machine, thus forming the lubrication layer on the protective coating of the magneto-optical disk.
Another example is disclosed in Japanese Laid Open Patent No. (Tokkai-Hei) 05-163564. In this method, a lubrication layer comprising a fluorocarbon lubricant containing fluorine having more than one active polar group and carbon as its main components is formed on the surface of the protective coating of a magneto-optical disk. The method includes the steps of applying the solution in which polyfluorocarbon lubricant is dissolved in isopropyl alcohol onto the surface of the protective coating which comprises an organic resin by the spincoat method; and heating the lubrication layer at the temperature of 80.degree. C. to evaporate the solvent. Thus, a lubrication layer is formed.
A third example includes a lubricant in the protective coating of the magneto-optical disk. According to Japanese Laid Open Patent No. (Tokkai-Shou) 64-043834, the structure is disclosed in which macromolecule sheet containing a lubricant comprising a fatty acid or ester or a mixture of them are adhesive-bonded to the protective coating. Moreover, according to Japanese Laid Open Patent No. (Tokkai-Shou) 63-098857, carbon fluoride lubricant is disclosed as a lubricant; according to Japanese Laid Open Patent No. (Tokkai-Hei) 03-160642, functionality silicone compound is disclosed as a lubricant; and according to Japanese Laid Open Patent No. (Tokkai-Hei) 04-209346 a lubricant comprises a perfluoropolyether having isocyanate end groups that is polymerized.
However, the magneto-optical disk manufactured by the above-mentioned three methods capable of the magnetic field modulation recording have some problems, for example, difficulties in manufacturing, and a lack of the durability of sliding.
In the first example, the solvent which is used in diluting the lubricant may react with the polycarbonate of the substrate during volatilizing, thus decreasing the transmissivity of the substrate. Thus, in order to prevent an explosion, an apparatus for collecting the volatilized solvent is necessary. Moreover, an apparatus for pressing is required, thus making it difficult to provide magneto-optical disk at low price.
As to the second conventional example, the magneto-optical disk requires the use of a heating apparatus for evaporating the solvent used for diluting the lubricant and an apparatus for collecting the evaporated solvent used for preventing the explosion.
In the third conventional example, a lubricant of fatty ester, lubricant of carbon fluoride, and lubricant of functionality silicone compound are mixed. The magneto-optical disk of this example can initially decrease the frictional force, but the durability of sliding cannot be maintained due to the increase of the frictional force under the high temperature and high humidity environment.